1. Technical Field
The invention relates to a lens manufactured by the mold molding and, more particularly, a molded lens having micro lens diameter such as an imaging lens for a mobile terminal, a recording/reproducing lens for an optical disk, a projecting/receiving lens for optical communication and an objective lens for an endoscope.
2. Description of the Related Art
In recent years, the molded lens is utilized in various uses at the request of weight reduction and cost reduction.
For example, in the case where the molded lens is formed by the injection molding, the molded lens is formed by a series of the following operation processes. That is, plastic material is softened by heating, the material is injected into the mold at a high pressure to transfer an optically transferred surface profile of the mold to the material, the mold is cooled, and then the molded lens is taken out by opening upper and lower portions of the mold.
Meanwhile, it is desired that the optically transferred surface profile of the mold should be transferred satisfactorily to a product lens over the entire surface. However, in the periphery of a lens surface, a molding pressure is hard to apply and thus it is difficult to satisfactory transfer the surface profile of the mold.
On the contrary, when a molding pressure is increased such that a sufficient pressure is applied to the periphery of the lens surface, burr occurs easily.
Also, when a mold clamping force is increased to prevent an occurrence of the burr, it is hard for air to come out of the mold. Thus, air still remains between the mold and the lens surface. As a result, a surface of the mold may not be transferred.
Therefore, in related art, a difference between an outer diameter of the lens surface and an effective aperture of the lens surface is set so large that deterioration of transferability is not caused within the effective aperture of the lens surface.
JP 2001-341134 A (corresponding to US 2001/0053395 A) discloses technology to process a lens optical surface with high precision.
However, with the progress of mobile devices such as a cellular phone, nowadays the demand for size reduction of the imaging lens becomes extremely strong. Thus, an outer diameter of 5 mm or less is going to become the mainstream as an outer diameter of the lens surface of the imaging lens. Further, the demand for an outer diameter of 1 mm or less also becomes stronger.
In such micro lens, it is difficult to utilize JP 2001-341134 A. Also, even though JP 2001-341134 A is used, it is difficult to improve transferability in the peripheral portion of the mold without fail. As a result, unless an effective aperture of the lens surface is set considerably smaller than an outer diameter of the lens surface, a desired refracting action cannot be exerted on rays passing through the peripheral portion of the lens.
Therefore, in order to maintain the optical performances satisfactorily in such situation, the effective aperture of the lens surface is set considerably small. Conversely, in order to ensure the effective aperture of the lens surface having a predetermined size, the diameter of the lens surface is set large, which conflicts with the above demand for size reduction. In this event, such problem arises in not only the molded lens manufactured by the injection molding method but also molded lens manufactured by the compression molding method.